Fibrous composites are promising lightweight, high-strength materials which retain their strength at high temperatures. The fibers contribute high tensile strength to the refractory matrix and also impart resistance to crack propagation. The high toughness of such composites has been characteristically associated with limited bonding between the fibers and the matrix in order to give substantial fiber pull-out as a major source of toughening. On the other hand, brittle failure in unsuccessful composites and resultant flat fractures with essentially no fiber pull-out have been attributed to strong fiber-matrix bonding.
Ceramic fiber-ceramic matrix composites offer unequaled high temperature performance and stability in oxidizing environments; however, fabrication of strong, tough composites is a difficult task. For purposes of this application, the adjectives ceramic and refractory are used generally interchangeably. A key factor in determining the properties of the composite is the nature of the interface between the matrix and the fiber. Fiber coating provides an effective and versatile method for controlling the fiber-matrix interface properties. Fiber coating also protects the fibers from degradation during the handling and composite fabrication process.
Good toughness characteristics can be introduced into composites with refractory fibers and a refractory matrix only when there is very limited bonding between the fibers and matrix, and between adjacent fibers. However, many possible combinations of fiber and matrix result in strong bonding over most, if not all, of the range of practical processing conditions, thereby limiting the potential toughness ranges of these composites. Such a problem exists, for example, with oxide-based fibers, because they have a tendency to degrade or react with the matrix when the refractory materials are molded. The reaction between the fiber and the matrix results in a high degree of bonding which renders the toughening mechanism inoperable. This problem for oxide-based fibers can be solved by creating a barrier coating on the fibers. Preferred barrier coatings are relatively inert and serve to physically separate the continuous phase refractory matrix and the reinforcing fibers. Such a barrier eliminates reactions between the oxide fibers and the refractory matrix or at least substantially slows the kinetics of such reactions so that toughening occurs in the composite. However, the adhesion of such an oxide coating to the associated fiber substrate for composite applications must be satisfactory to withstand the stresses to which the coated fibers are subjected in forming the composite structure.
While there are many methods for creating oxide and other refractory coatings on such fibers, none is considered completely suitable.
The traditional processes for fiber coating are chemical vapor deposition (CVD) and dip coating. CVD coatings typically must be performed at very low pressures and at moderate-to-high temperatures to achieve adequate infiltration of the tows. The by-products and/or reactants are often corrosive compounds such as hydrochloric acid. As a consequence, CVD fiber coating is complex and expensive. Dip coating in a preceramic polymer solution or a sol-gel solution is less expensive than CVD, but other problems are encountered. Primary obstacles to be overcome include nonuniformity throughout a multifilament tow, bridging between fibers, and tracking. Tracking is the preferential deposition of coating between fibers due to capillary effects during drying that does not result in bonding the two fibers together. Bridging is the bonding of adjacent fibers by the coating. Because of the complexity of CVD coating processes and because of the quality of the coatings obtained from traditional dip coating processes, new methods of fiber coating are being explored.
Accordingly, new coating processes for providing smooth, substantially uniform oxide coatings on continuous multifilament refractory fiber tows, and particularly on fabrics woven from such fiber tows, without requiring the use of expensive chemicals and/or processing steps are desirable.